Question

1988 Buick LeSabre cranks but won't start.

'88 LeSabre LSF with the 3.8 V6

Was running but was hard to get started and idled really rough.

I had it idling in the driveway 'cause my mom was about to run some errands. I decided to shut it off to see if it was easier to start once it was warmed up. It wouldn't start, didn't even sound like it was firing at all.

It cranks good.

It's getting fuel to the injector rail but not out of the return side (nothing is coming through the pressure regulator).

When I pop the top of the air cleaner off and spray starting fluid in toward the injector it will run.
The pump is new and I can hear run when I turn the key.
Fuel filter is new.

Is it fuel pressure? Do I need a special gauge to check fuel pressure? (It has a port on the rail).
What else might it be? ECM?